This invention relates generally to the purification of nickel sulfate by solvent-extraction and specifically to extraction of nickel sulfate from residue resulting from regeneration of spent electrolyte in copper electrorefining.
In the electrorefining of copper, electrolytic corrosion of impure copper anodes releases associated impurities. As high purity copper plates-out at the cathode, those impurities which are less noble than copper are dissolved in the electrolytic solution of sulfuric acid and copper sulfate, while more noble impurities settle to the bottom of the refining tank as anode slime. Typically, the electrolyte is continuously bled-off and made-up at a rate sufficient to maintain the impurity concentration within a maximum acceptable limit. This bleed stream is treated by stagewise evaporative crystallization to recover sulfuric acid leaving a residue containing a large portion of nickel sulfate. As nickel sulfate has substantial market value, the present invention is concerned with the extraction of high purity nickel sulfate from this waste residue.
In the secondary copper refining industry, this residue typically is composed principally of nickel sulfate due to the prevalent association of nickel-containing scrap with copper-containing scrap and is frequently referred to as "crude nickel sulfate". A typical assay of such residue contains roughly 70% metallic sulfate which in turn is roughly 80% nickel sulfate.
Conventional methods for treating this crude nickel sulfate first solubilize the residue in acid, then precipitate impurities leaving nickel sulfate in solution from which it is finally crystallized. The major disadvantage in this approach is that impurities are separated from the nickel sulfate (rather than separating the nickel sulfate from the impurities), thus requiring especially sensitive process control during impurity precipitation if high recovery and high purity is to be realized for the end product.
In contrast, the present invention provides a less sensitive alternative treatment whereby nickel sulfate is directly separated from associated impurities utilizing methods of selective leaching and pH regulated solvent-extraction to recover high purity nickel sulfate. An additional advantage resulting from this approach is that regeneration of the leach solution is made practical.
Of general interest concerning leaching of nickel sulfate are ammoniacal leaching techniques for metal sulfates, such as is disclosed in the article "Chemistry of the Ammonia Pressure Process for Leaching Ni, Cu, and Co from Sherritt Gordon Sulphide Concentrates", Journal of Metals, March 1955, hereby incorporated by reference, wherein various metal salts are selectively leached via metal ammine complexing.
Of general interest in the present invention concerning solvent-extraction of nickel sulfate is the process briefly described in the article "New Recovery Process Can Yield both Electrolytic Nickel and Copper", Engineering and Mining Journal, 94, January 1972, wherein copper and then nickel are serially extracted from spent copper-refining electrolyte utilizing General Mills' LIX-64N (.TM.) organic extractant. Also of general interest as another example of use of this type of extractant is the extraction process disclosed in U.S. Pat. No. 3,794,719 "Reduction of Nickel Contamination by Zinc in Alpha-Hydroxy-Oxime Extraction" issued Feb. 26, 1974, wherein coextraction of zinc is suppressed by a multistage extractant loading procedure.